1. Field of the Invention
The present invention relates to a power tool which can be driven and rotated by a motor and, specifically, the invention relates to a power tool which is enhanced in durability and operation efficiency due to the improved cooling mechanism of the motor.
2. Description of the Related Art
As a power tool for fastening a screw, a bolt and the like, there is known an oil pulse tool which can generate a striking force using oil pressure. In the oil pulse tool, there is no collision between metals. Therefore, when compared with an impact tool of a mechanical type, the oil pulse tool has a characteristic that the operating sound thereof is low. As this type of oil pulse tool, for example, there is available a technology disclosed in JP-2005-040881-A which uses a motor as a power source for driving an oil pulse unit and also in which the output shaft of the motor is directly connected to the oil pulse unit. Since the oil pulse unit rises in temperature as it is used, there is interposed a fan between the motor and oil pulse unit (on the front end side of the motor); and, the motor can be cooled by the fan. When pulling a trigger switch which is used to operate the oil pulse tool, a drive current is supplied to the motor. In JP-2005-040881-A, there is interposed a reduction gear between the rotation shaft and output shaft of a motor, and necessary output torque is secured by driving a small-size motor at a high revolution, thereby reducing the size of the product, that is, the oil pulse tool.
In an ordinary power tool, there is interposed a reduction gear between the rotation shaft and output shaft of a motor, and necessary output torque is secured by driving a small-size motor at a high revolution, thereby reducing the size of the product, that is, the power tool. In an oil pulse tool, there is used oil pressure for generating a striking force and the rotation force of the motor is applied suddenly at a certain angle to a leading end tool which is mounted on the output shaft of the motor. In the striking operation, the tool receives a reaction force from the leading end tool side and this reaction force is applied to the support portion of a reduction gear; and, therefore, when a reduction gear is provided in the oil pulse tool, the reaction force becomes large, which increases vibrations in the striking operation. Thus, in order to reduce the vibrations in the striking operation, there is proposed a direct drive mechanism in which no reduction gear is interposed between the rotation shaft of the motor and oil pulse mechanism.
In order to employ the direct drive mechanism, it is necessary to use a motor of a type that provides a low speed and high torque. Generally, when compared with a high speed low torque type of motor using a reduction gear, the low speed high torque type of motor is large in size. Also, when the low speed high torque type of motor is used, it is necessary to sufficiently secure the strength of a bearing portion for supporting the rotor of the motor. Especially, during use of a tool using such motor, when there occurs a state different from the original use object of the tool (such as drop), if the strength of the rotor support portion is insufficient, there is a possibility that the tool can be broken due to the inertial force of the rotor. Therefore, the rotor support portion must be structured such that the two ends thereof secure sufficient strength respectively.
In the oil pulse mechanism, after striking, due to the action of the reaction force from the leading end tool side, the number of revolutions of the oil pulse unit is reduced; and, in a brushless dc motor including a direct drive mechanism, due to no provision of the reduction gear, the number of revolutions of the motor is also reduced. Suppose the brushless do motor is used, when the number of revolutions of the motor is reduced due to the reaction force, there is a possibility that a large current can be generated in a drive circuit to thereby raise the temperature of a switching element abnormally.